Cercospora leaf spot is one of the most important, globally prevalent leaf diseases of plants from the species Beta vulgaris and Spinacia oleracea. It is caused by the fungus Cercospora beticola. Plants infested by this disease typically form small, relatively round leaf spots (2-3 mm) that are light gray in the middle and are surrounded by a red-brown border. In a severe infestation, the leaf spots overlap, so that entire portions of the leaf blade dry out. Small black dots (pseudostromata) are visible within the fully formed spots, and a gray, felt-like covering (conidia bearers with conidia) forms under damp conditions—predominantly, on the leaf underside. Severely infested leaves first turn yellow, then turn brown and die. New leaf growth occurs in parallel, wherein the leaves become diseased again and die, however. At first, damage symptoms only on individual plants are visible; however, with spread of the disease, formation of persistent infestation nests often occurs. The further propagation over the entire field takes place via rain and wind.
The pathogen Cercospora beticola was first described in the second half of the 19th century, in Italy. Up to 40% crop losses may occur due to a severe infestation, which may be triggered by humid weather, early row closure, a high infection potential from previous years, or strong irrigation. These losses result from a reduced beet crop and reduced sugar content; see Holtschulte ((2000) “Cercospora beticola—worldwide distribution and incidence,” pp. 5-16, in “Cercospora beticola Sacc. Biology, Agronomic Influence and Control Measures in Sugar Beet,” vol. 2 (M. J. C. Asher, B. Holtschulte, M. R. Molard, F. Rosso, G. Steinrücken, R. Beckers, eds.). International Institute for Beet Research, Brussels, Belgium, 215 pp.). In order to fight back against the disease, intercropping or fungicides are often used. A chemical control of Cercospora beticola via fungicides incurs costs to the farmer and pollutes the environment. Repeated applications of fungicides additionally increase the selection pressure on fungicide-tolerant Cercospora beticola strains. This is contrary to a sustainable agricultural practice. It is worth mentioning that during the last few years stems of Cercospora beticola occurred which showed resistance against one or more fungicides; see for Trkulja, Nenad R., et al. “Molecular and experimental evidence of multi-resistance of Cercospora beticola field populations to MBC, DMI and QoI fungicides.” European Journal of Plant Pathology 149.4 (2017): 895-910. The problem became such severe that the German Federal Office of Consumer Protection and Food Safety (BVL) approved the exemptional admission of copper-based fungicides for combating Cercospora. However, copper-based fungicides are generally regarded (depending on the dosage) as harmful for humans and environment. Copper is a heavy metal which may accumulate in the soil.
Indirect combat is done via the selection of cultivars with healthy leaves and cultivation of the beets with at least a 3-year crop rotation. Markedly better control of the infestation may be achieved with a combination of tolerant or resistant cultivars. Less susceptible Cercospora-tolerant beet cultivars have been offered on the market since 2000 (Steinrücken 1997, “Die Ziichtung von Cercospora-resistenten Zuckerrüben.” [“The breeding of Cercospora-resistant sugar beets.”], Vorträge für Pflanzenziichtung [Lectures on Plant Breeding], Volume 37, Lecture symposium, Mar. 4-5, 1997, Kiel). These cultivars are furnished with a quantitative resistance to Cercospora beticola. The resistance of these cultivars is based upon several genes and is quantitatively passed down, wherein the exact number of the genes that are responsible for the resistance is not known; see Weiland and Koch (2004), Sugarbeet leaf spot disease (Cercospora beticola Sacc.), The Plant Journal, 5(3), 157-166. The complex quantitative heredity was confirmed via several Quantitative Trait Loci (QTL) analyses. This method allows the mapping of polygenic inherited resistances and is a reliable technique for identifying the number and position of genetic resistance factors on the genetic linkage map of a host plant. In this way, multiple causative QTL's could be determined on each chromosome of the sugar beet.
The mappings were performed with different Cercospora-resistance donors, wherein the observed QTL effects were, for the most part, small. The maximum declared phenotypical values were at 5%.
In continuative studies, lists of differentially expressed genes have been described. In a study by Weltmeier et al., ((2011) Transcript profiles in sugar beet genotypes uncover timing and strength of defense reactions to Cercospora beticola infection, Molecular plant-microbe interactions, 24(7), 758-772), a genome-side expression profile for various genotypes of sugar beet (i.e., Cercospora-resistant, -tolerant, -susceptible, etc.) was created with the aid of a microarray-based technology during the pathogen infection in order to analyze transcriptional changes in the expression profile in connection with leaf spot. Via these analyses, the authors were in a position to create a pathogen-induced transcription profile in various tested genotypes of sugar beet and to determine potential candidate genes. However, these genes have not yet been characterized in detail. The genetic and functional background of Cercospora resistance and the identity of the resistance genes have until now been entirely unclear.
However, with the quantitative heredity of QTL, not only is the desired resistance to Cercospora beticola introduced into the plant, but, rather, often unwanted features as well, such as, for example, reduced yield, due to the inheritance of additional genes that are linked with the positive feature of Cercospora resistance. This phenomenon is also known by the term, “linkage drag.” Furthermore, the enormous breeding cost that is required in order to select for multiple resistance loci without thereby reducing the yield may have negative effects on the vitality of the plants; see Weiland and Koch, 2004.
Breeding companies have offered Cercospora-tolerant cultivars on the market for more than a decade. The resistance of these cultivars is inherited via multiple resistance genes with small effect. However, a disadvantage of these cultivars consists in the cultivar development being very laborious and complicated due to the complicated heredity, and in such cultivars having a markedly poorer yield performance relative to normal cultivars, in the absence of an infestation. Among other things, this may be linked to the epigenetic interaction of some resistance genes with genes that are responsible for sugar production, which leads to reduced fitness of the plants, in the absence of the pathogen. Furthermore, Cercospora shows the tendency to overcome the tolerance of long-established cultivars. Moreover, the so far available resistance scores of non-adapted, wild genetic resources is usually not reliable and not comparable among each other as the underlying studies took place at different environment conditions, under different infestation pressure and with different pathogenic stems of Cercospora. In this regard it should be mentioned that environmental parameters like moisture, temperature, wind etc. (which tend to be unstable) have significant influence on the progress of the Cercospora disease after infection. It is common that a specific genetic resource shows high level of tolerance/resistance in one study and tends to be completely susceptible in another study. Due to the above given factors it was so far not possible to identify a dominant resistance gene having a major effect towards Cercospora although there is a strong demand for such a gene which could be easily transferred into already existing cultivars and varieties to establish resistance towards Cercospora. 
The use of new breeding techniques based upon gene editing, e.g., by means of TALE nucleases or CRISPR systems, and of transgenic approaches, is not applicable on the so far available genetic material due to the complicated heredity and the multitude of the genes which are involved in the resistance development, the majority of which have not yet been identified and characterized.
For sustainable breeding against Cercospora leaf spot that is to counteract the danger of Cercospora variants that overcome resistance, it is necessary to continuously identify new resistance genes and integrate these into the gene pools of cultivated plants such as sugar beets. In particular, the aim consisted in the provision of suitable resistance genes that, after expression in the plant, on their own already produce a very large, dominant resistance effect against Cercospora beticola. According to the invention, this aim is achieved via the embodiments characterized in the claims and in the specification.